1. Field of the Invention
The present invention relates to a high frequency digital synthesizer with aperiodic correction optimizing the spectral purity.
It applies essentially to the construction of frequency shift transmitters-receivers, modulators-demodulators for digital transmission systems as well as the construction of radiography systems using nuclear magnetic resonance. In these applications, synthesis of the clock signals is obtained by means of frequency synthesizers having very brief frequency acquisition times and very good frequency resolutions. Only the frequency synthesizers using direct synthesis methods allow such results to be obtained.
As counterpart to the high speeds obtained by the direct digital synthesis method, the clock signals delivered by the corresponding frequency synthesizers have reduced spectral purity because of the presence of parasite spectral lines of energy -50 to -60 dBc and high noise. In addition, the frequency synthesizers using this method have a relatively low maximum operating frequency of the order of a few megahertz. Now, for numerous applications, these performances appear distinctly insufficient and do not in particular allow the problems of proximity in radiotelecommunications to be solved. Furthermore, in order to keep a high synthesis speed in high frequencies (HF, VHF, UHF) the addition and recopying loops must have wide bands and are for this reason inefficient as filters. Since any synthesized frequency leads to a spectrum of parasite spectral lines, the frequency synthesizers using direct synthesis methods are provided with devices for attenuating both the noise and the amplitude of the parasite spectral lines.
2. Description of the Prior Art
A device of this type is described for example in the U.S. Pat. No. 4,185,247. The direct frequency synthesizer which is described therein includes an accumulator driven by a clock frequency which is as high as possible with respect to the frequency to be synthesized. This latter is provided by the pseudo periodic overflow of the accumulator. Such pseudo-periodicity gives a parasite phase modulation which is known and which is formed by the residue of the accumulator expressed as a fraction of the synthesized clock. This, using a digital-analog converter, allows a reverse modulation of the parasite modulation to be provided and so the parasite spectral lines to be reduced. These results are obtained with a phase correction system which is formed either from a tuned phase shift, containing variable capacity diodes known under the name of "varicaps" or from high speed loops containing a voltage controlled oscillator into the loop of which the correction modulation is injected. These phase correction systems have as drawbacks the fact of having poor linearity, of not being a periodic and of not being climatically stable and, furthermore, they do not allow frequencies to be synthesized beyond the octave with a linearity less than 1. From the plan of the construction described in the above U.S. patent, the maximum synthesizable frequency seems to be half that of the digital-analog converter used for providing the correction. Furthermore, the adders, counters and accumulators used are such that their speed decreases when their size increases, that is to say when the resolution or the synthesis pitch becomes finer and finer.
In other variants of construction described, for example in the patent GB No. 2 026 268 or else in the French patent No. 2 431 800, the level of the residual uncorrected spectral lines is attenuated by modulating the phase increment of the accumulator on the digits not corrected by the digital-analog converter. But this attenuation is provided to the detriment of the noise and in any case the same modulation power (spectral line noise) is to be found at the output of these devices.